Hypertrophy Rep Range: Why High Reps With Low Weights Builds Muscle

What builds the most muscle? Should you do more reps with lighter weights? Or are heavier weights and low reps the way to go?

Long story short, both high reps with lighter weights AND low reps with heavier weights do a similar job of building muscle.

That is, you’ll see similar levels of growth with sets of 5 reps, sets of 15 reps, or sets of 30 reps.

You’ll gain more strength with low reps and heavier weights. But if it’s more mass that you’re after, high reps and light weights or low reps and heavy weights will do a similar job.

Today, I want to take a closer look at the research that shows similar gains in muscle size across a wide spectrum of loads, ranging from light to medium to heavy.

Let’s jump right in.

Why Training With High Reps and Light Weights Builds Muscle

Conventional wisdom has it that weight training with high repetitions and light weights builds muscular endurance, but makes little contribution to gains in muscle mass.

Heavy weights and low reps has long been the accepted “best way” to maximize muscle growth.

Back in 2010, Stuart Phillips, a kinesiology professor at Canada’s McMaster University, began to overturn much of that conventional wisdom.

Phillips and his team found that muscle protein synthesis, a key driving force behind muscle growth, was higher with light weights and high repetitions (4 sets of 24 reps) than it was with heavier weights and lower reps (4 sets of 5 reps) [6].

At the time, these findings generated a lot of controversy.

They were also dismissed by many, mainly on the basis that the study looked at short-term changes in protein synthesis rather than long-term gains in muscle size.

So, Phillips set up another trial.

This time, he got a group of guys to train their legs on the leg extension machine three times a week for 10 weeks, using one of three different set and rep configurations [2]:

• 1 set of 10-12 reps performed to voluntary failure
• 3 sets of 10-12 reps performed to the point of fatigue
• 3 sets of 30-40 reps performed to the point of fatigue

The result?

The amount of new muscle added to both legs was almost identical.

Resistance training with light weights and high repetitions stimulated just as much muscle growth as heavy weights and lower reps.

What’s more, the average size of both the fast twitch and slow twitch muscle fibers increased equally with heavy and light loads, meaning that both fiber types were recruited and stimulated during training.

Do High Reps Build Muscle in Advanced Lifters?

Once again, this study attracted criticism, most notably because it used untrained beginners as subjects.

Take someone who’s never done any weight training and get them to lift weights for a few months. They tend to grow no matter what they do.

Are you going to see the same results in guys who have been strength training for a few years?

To find out, Phillips recruited a group of men with an average of four years strength training experience. The men were assigned to one of two groups [8].

• Group one did 20-25 reps per set.
• Group two did 8-12 reps per set.

The findings?

After 12 weeks of resistance training, there was no statistically significant difference in the rate of muscle growth between the two groups.

As with the research in novices, the average size of both the slow twitch and fast twice muscle fibers increased to a similar extent in both groups.

But, that doesn’t mean the two protocols delivered identical results.

The average amount of muscle mass gained in the high rep group was 2.2 pounds (1 kilogram), compared with 3.5 pounds (1.6 kilograms) in the low rep group.

Had the study lasted longer, or the number of subjects been larger, the difference in results between the two groups may have become large enough to cross the statistically significant threshold.

Low Reps/Heavy Weights vs High Reps/ Light Weights

Of course, these are the results from just a few studies. And drawing conclusions about anything from two or three studies is never a good idea.

However, there’s plenty of other research out there showing multiple benefits of weight training in a higher rep range.

• Light slow-speed training (55-60% of one-rep-max, 3 seconds to lift and lower the weight) has been shown to increase both muscle mass and maximal strength [3]. The results are comparable to those obtained with heavy normal-speed strength training (80-90% of one-rep-max, 1 second to lift and lower the weight).
• Both heavy (4 sets of 8-10 reps) and light training (4 sets of 18-20 reps) activate the expression of various genes involved in muscle growth [4].
• Eight weeks of training the arms with light weights (20 rep-max) and short (30 seconds) rest periods led to gains in muscle size that were no different to those seen with heavier weights (8 rep-max) and longer (3 minute) rest periods [1].
• Low-intensity training not to failure also stimulates protein synthesis in connective tissue just as well as heavy training, giving it a role during injury rehabilitation to improve regeneration of connective tissue [7].
• Eight weeks of training with high reps and light weights (30-40 reps per set) builds just as much muscle as low reps (8-12 reps per set) and heavy weights [10].

Super High Rep Training: How Light is Too Light?

Most of the studies we’ve looked at show similar rates of muscle growth with both low intensity (light weight, high rep) and high intensity (heavy weight, low rep) training.

But what happens when you start using super high rep training? And by super high, I’m talking about 60-70 reps per set?

That was the question asked by a Brazilian research team, who took a group of 30 untrained men and got them to lift weights twice a week for 12 weeks [9].

The lifters were split into three groups. All three groups trained the biceps and quads on one side of their body with very light weights and super-high reps – 20% of their one-rep max and 60-70 reps per set.

On the other side of their body, the men used one of three different loading protocols: a high-rep protocol, where the men lifted at 40% of their one-rep max for around 30 reps per set, a moderate-rep protocol, where the men lifted at 60% of their one-rep max for 15-20 reps per set, and a low-rep protocol, where the men lifted at 80% of their maximum for 10-15 reps per set.

All three groups trained to failure, and did two exercises: the biceps curl and leg press.

At the end of 12 weeks, training with low, moderate and high reps all led to similar gains in muscle size. High reps triggered just as much muscle growth as heavy weights and lower reps.

However, it was a different story for the side of the body that was trained with super-high reps, where muscles grew at half the rate they did in the other three protocols.

In other words, while sets of 30 reps led to gains in size that were on par with sets in the 10-15 rep range, training with just 20% of your one-rep max appears to be below the threshold needed to maximize hypertrophy.

High Reps vs Low Reps: Two Types of Muscle Growth

Some say that the type of muscle growth caused by training with lower weights and higher reps isn’t as “good” as the muscle gained with heavier weights.

Here’s what they’re talking about.

If you could take a closer look at a slice of muscle tissue, you’d see that it’s made up of many smaller muscle fibers. Your muscles get bigger when these individual fibers become thicker, a process known as hypertrophy.

Inside each fiber are rod-like structures called myofibrils, which run parallel to one another. Myofibrils are the part of the muscle that contribute to force production.

There is also a fluid part of the muscle fiber, known as the sarcoplasm, in which the myofibrils are embedded. It’s filled with stuff – such as water, glycogen, and myoglobin – that doesn’t contribute directly to the production of muscle force.

The term myofibrillar hypertrophy refers to an increase in the volume of the myofibrils, while sarcoplasmic hypertrophy describes the expansion of the sarcoplasm.

The idea is that training with higher reps leads mainly to so-called “non-functional” sarcoplasmic hypertrophy, creating “puffy” muscles that tend to deflate relatively quickly.

Training with heavy weights and lower reps, on the other hand, is said to preferentially increase the rate of myofibrillar hypertrophy, leading to dense, strong, “functional” muscles.

That’s the theory, anyway. Personally, I’m not convinced.

In one of the few studies to compare the effect of high versus low repetitions on myofibrillar and sarcoplasmic protein synthesis, a low-intensity weight training routine did increase the rate of sarcoplasmic protein synthesis to a greater extent than heavier lifting [6].

But, it increased the rate of myofibrillar protein synthesis too.

Interestingly enough, when it was measured 24 hours after exercise, high reps and light weights (4 sets of 24 reps) increased myofibrillar protein synthesis to a far greater extent than low reps and heavy weights (4 sets of 5 reps).

Granted, this study looked at short-term changes in protein synthesis after exercise, rather than long-term changes in muscle tissue after several months of training. We can’t assume that the former is a completely dependable way to predict the latter.

But it does paint a big question mark next to the idea that training with higher reps will preferentially increase sarcoplasmic rather than myofibrillar hypertrophy.

Does the sarcoplasm exist? Yes.

Can it grow? Yes.

However, there’s no compelling evidence to suggest that the hypertrophy produced by training with higher repetitions comes predominantly from “non-functional” sarcoplasmic hypertrophy, while the myofibrillar component of the muscle remains the same size, or grows to a much lesser degree.

Are High Reps Better for Hypertrophy?

None of this means that training with light weights and high reps is now the official “best way” to train for hypertrophy.

The fact that it’s possible to gain muscle using higher reps and lighter weights doesn’t necessarily mean that it’s a good idea to do so. High reps aren’t necessarily better, they’re just another option on your training menu.

Remember, there were no discernible advantages to training with high reps and light weights. They didn’t lead to superior gains in size or strength. But each set took twice as long to complete.

Training to failure in a higher rep range is also highly unpleasant and extremely painful – a lot harder than lower reps and heavier weights.

Doing longer, more painful workouts simply to generate the same results doesn’t sound like a great idea to me.

Plus, lower reps and heavier weights still win the day as far as building strength is concerned [5]. That is, you’ll see bigger strength gains with sets of 3-5 reps, for example, than you will with sets of 15-20 reps.

Adding Reps vs Weight: Which is Better?

You’ve probably come across the concept of progressive overload, which refers to the idea that you need to constantly increase the demands you impose on your muscles in order to stimulate growth.

To a lot of people, overload means increasing the amount of weight that’s on the bar when you lift it.

However, the fact that higher reps are an effective way to stimulate muscle growth, and not just muscular endurance, means that doing more reps can also be used to apply the overload principle.

That is, you don’t have to keep on lifting more and more weight to make your muscles grow. Sometimes you’ll be better off lifting the same amount of weight while aiming to increase the number of reps you do.

Depending on the exercise you’re doing and the equipment you have available, adding weight isn’t always a realistic option.

To go from squatting 200 to 205 pounds, for instance, represents a 2.5% increase in weight. But if you’re doing lateral raises with a 15-pound dumbbell, adding five pounds is a 30% jump in weight. Such a large increase in load is likely to push you outside of whatever rep range you were working in.

In this case, you’re better off trying for more reps with the same amount of weight. That is, if you did 12 reps in your last workout, aim for 13 or 14 the next time.

In a 2022 study, US researchers report that doing more reps worked just as well for muscle growth as adding weight [7].

Subjects taking part were assigned to one of two groups. Both groups trained their legs twice a week, doing the same exercises, for the same number of sets.

The only difference was in the progression model used by the two groups.

1. Group one added weight while keeping the rep range constant.
2. Group two added reps while keeping the weight constant.

After 8 weeks of training, adding reps was at least equally effective as adding weight for increasing muscle size. In fact, doing more reps led to slighter greater gains in rectus femoris (one of the quadricep muscles) compared to adding load.

Lifting more weight did a slightly better job of increasing maximal strength, while both strategies were equally effective for muscular endurance.

Here’s how the researchers sum up their findings:

“Overall, these results suggest that, from a hypertrophy standpoint, progressions can be made with load, repetitions, or conceivably a combination of the two over the course of an 8-week training block. The results are generally consistent with the body of literature, which shows similar hypertrophy across a wide spectrum of loading ranges.”

What Rep Range Is Best for Building Muscle?

To build muscle, most of your training should be done in the 5-15 rep range. That’s heavy enough to put plenty of tension on the muscle, but not so heavy that you can’t control the weight. If I had to pick a single “best” rep range for building muscle, it would be 5-15.

On paper, both high and low reps can be used to gain muscle. But in practical terms, you’re usually better off staying in that 5-15 range.

Why is that?

Have you ever tried doing a set of 20-rep squats to the point where you haven’t got any more reps left in the tank?

It’s brutal.

Your legs are on fire, your lungs are locked in a desperate battle for oxygen, and you feel like throwing up.

Doing multiple sets of 20–40 reps for compound lifts like squats and deadlifts just isn’t feasible.

Even on isolation lifts, training with lighter weights and higher reps is generally more painful than moderate reps. It’s not that it can’t work. The research clearly shows that it does.

It’s just extremely painful, and most people are going to be limited in those sets by their willingness to tolerate pain and discomfort.

On the flip side, while very heavy weights that limit you to less than a handful of reps per set can be used to gain muscle, they also have their limitations.

For one, fewer reps per set will typically require more sets to stimulate the same amount of growth.

There was an interesting study published in the Journal of Strength and Conditioning Research, where researchers tested three different set and rep configurations during a bench press training program – 7 sets of 4 reps, 4 sets of 8 reps, and 3 sets of 12 reps [6].

After 10 weeks, MRI scans of the pecs revealed no significant difference in the amount of muscle growth between the three groups.

While strength gains were largest with lower reps and heavier weights, subjects doing 3 sets of 12 reps built the same amount of muscle as those doing 7 sets of 4 reps. However, the 3-set group spent a lot less time in the gym. They gained the same amount of muscle with fewer sets.

Lots of heavy sets can also be very painful on your joints, especially if you’re in your 40’s, 50’s or beyond.

The 5-15 rep range sits between those two extremes, which is why it works so well for building muscle. If you go much heavier, your workouts end up taking longer. Go lighter, and the whole experience becomes a lot more painful than it needs to be.

However, you don’t have to pick just one rep range. In fact, the range of repetitions you can use to build muscle is a lot wider than previously thought.

Most studies out there show very similar gains in muscle size across a variety of rep ranges. That gives you a lot more choice about the type of training you do.

For example, joint issues or injuries may mean that lifting heavy weights causes pain in your shoulders, elbows, knees or wrists.

The solution is very simple. If going heavy on certain exercises causes you pain, just go light instead.

You can make the switch from heavy weights and low reps to low weights and high reps without missing out on any gains.

Maybe you just prefer using lighter weights on certain exercises, and heavier weights on others.

Again, you can do so quite happily without worrying that you’re putting the brakes on muscle growth.

In summary, training with high reps and light weights does a lot more than improve muscular endurance – it’s also going to make your muscles grow.

Training with heavy weights is certainly the way to go if building strength is a priority.

But as long as you train hard and push yourself, taking your work sets close to failure, muscle growth is very similar across weights and rep ranges.

Frequently Asked Questions

Do high reps and low weights work better for weight loss?

When they’re trying to lose body fat, many people will make changes to their training program, switching to high reps and light weights, and shortening the amount of rest they take between each set, all in the hope that doing so will burn more calories and lead to a faster loss of body fat.

In most cases, this is a mistake.

When fat loss is the main objective, the basic structure of your training program should be much the same as it is when you’re trying to add mass. The big difference lies not in the type of training you’re doing, but in your overall calorie intake.

Gaining muscle can be done effectively with low, medium and high reps, and the same holds true for fat loss.

How do you determine which exercises should have high vs low reps?

There’s no rule saying this exercise has to be done with high reps or that exercise should be done with low reps.

In general, compound exercises like the bench press, squat and deadlift are better suited to heavier weights and lower reps.

Single-joint isolation exercises, on the other hand, are typically done in a higher rep range.

However, that’s just a general guideline. There’s nothing to stop you doing high-rep sets of squats, bench presses or any other compound movement, just as long as you’re able to maintain proper technique throughout the set.